function htmlFontSize(){
    var clientWidth = document.documentElement ? document.documentElement.clientWidth : document.body.clientWidth;
    
    if(clientWidth >= 768 && clientWidth < 1280){
    clientWidth = 640;
    }else if(clientWidth >= 1280){
    clientWidth = 640; 
    }
    
    document.documentElement.style.fontSize = clientWidth * 1/10+"px";
    return clientWidth * 1/10;
    }
    htmlFontSize()
    
    window.onresize = function(){htmlFontSize()}